1. Field of the Invention
The present invention relates to an antenna for radio device projecting outwardly from the case of a mobile telephone or the like, and a radio device using the antenna.
2. Description of the Related Art
In order to receive call-up signals, a mobile telephone and the like needs an antenna device which operates as an antenna at all times. On the other hand, it is not desirable for a long antenna to be projecting from the case at all times, since it renders the telephone less easily portable. Accordingly, various types of antenna devices have been proposed in which call-up signals are received by an antenna element having a relatively short physical length and projecting outwardly from the case at all times, a longer rod-like antenna element being extracted during communication to achieve high-gain reception and transmission. The present applicants have already disclosed an example of such an antenna device in Japan Patent Application 1996-266656.
The technology proposed in Japan Patent Application 1996-266656 will be explained simply with reference to FIG. 14 and FIG. 15. FIG. 14 is a circuit diagram showing the antenna for radio device described above in an extracted state. FIG. 15 is a vertical cross-sectional view of the extracted antenna for radio device shown in FIG. 14.
In FIG. 15, a mounting metal part 12 comprises a conductive metal and is securely screwed to a case 10 of a radio device. A through hole 12a is provided in the mounting metal part 12. In addition, a substantially cylindrical core 14 of insulating resin is provided coaxially to the mounting metal part 12 and extends outwardly therefrom. A first antenna element 16 is wrapped around the outer face of the core 14 so as to be substantially cylindrical. Furthermore, a cover 18 comprising insulating resin covers the first antenna element 16, and the bottom end of the cover 18 is secured to the mounting metal part 12. A hole 18a is coaxially provided in the cover 18, and a ring groove for clipping 18b is provided in the inner wall of the hole 18a.
The through hole 12a of the mounting metal part 12, the substantially cylindrical core 14, and the hole 18a of the cover 18 are provided around a single axis. Furthermore, a rod-like second antenna element 22 has a rod-like insulating member 20 on its tip, and is provided so as to be capable of moving freely in the same axial direction as the above. The outer face and base and the like of the second antenna element 22 are all covered by an insulating tube or the like. Furthermore, an axis-encircling groove for clipping 20a is provided in the tip of the rod-like insulating member 20, and a button 20b for restricting the movement of the second antenna element 22 in the storage direction is provided on the tip thereof. Furthermore, an axis-encircling groove for clipping 22a is provided in the insulator covering the base portion of the second antenna element 22. A step 22b is provided at the bottom of the second antenna element 22 to restrict its movement in the extraction direction. Furthermore, a ring for clipping 24 comprises a ring-shaped insulating resin having a cutaway portion, and is inserted into the ring groove for clipping 18b in the cover 18. When the antenna is stored, the ring for clipping 24 meshes with the groove for clipping 20a of the rod-like insulating member 20, thereby holding the antenna in the stored state. When the antenna is extracted, the ring for clipping 24 meshes with the groove for clipping 22a in the second antenna element 22, thereby holding the antenna in the extracted state.
As shown in FIG. 14, the first antenna element 16 comprises a zigzag portion 16a extending in the axial direction, and a folded portion 16b extending in the peripheral direction. The first antenna element 16 is wound cylindrically around the core 14, and the base of the folded portion 16a is conductively connected to the mounting metal part 12 by soldering.
A feeding metal part 26 comprises a conductive metal, and engages with the mounting metal part 12 screwed to the case 10. Further, the feeding metal part 26 is conductively connected via a cable 28 to an antenna input/output terminal 29 of a radio circuit 29.
According to the above constitution, when the antenna is extracted as shown in FIG. 14, the axial direction tip of the first antenna element 16 is connected at high frequency to the base of the second antenna element 22 by the coupling of a floating capacitance Ci, whereby the first and second antenna elements 16 and 22 function as a single antenna. Furthermore, when the antenna is stored (the diagram does not show this state), the second antenna element 22 is inside the case 10 and consequently is not connected at high frequency to the first antenna element 16, whereby only the first antenna element 16 functions as an antenna.
Accordingly, the antenna can be stored when the mobile telephone or the like is being carried, and is able to receive call-up signals using only the first antenna element 16. On the other hand, during communication, the second antenna element 22 is extracted, whereby the first and second antenna elements 16 and 22 enable the antenna to receive and transmit with high gain.
The previously proposed antenna for radio device described above is conveniently portable, and has high antenna gain during communication. However, when the second antenna element 22 is extracted, its base is coupled to the axial direction tip of the first antenna element 16 by the floating capacitance Ci, and this floating capacitance Ci is not always constant. During manufacturing, adjustment of the band of resonant frequencies at which the first and second antenna elements 16 and 22 function as a single antenna is complex.
Furthermore, when the second antenna element 22 is extracted, its base is capacitance-coupled to multiple axial direction tip portions of the folded portion 16b which extends in the peripheral direction of the first antenna element 16. Multiple closed loops are formed by the floating capacitances between these axial direction tip portions, and the partial impedance of the folded portion 16b. These multiple closed loops have a detrimental effect of lowering the antenna gain. There is a further problem, in that the resonant frequencies of the closed loops are not constant.